Expansins

Expansins are a family of plant cell wall–loosening proteins that promote turgor-driven cell expansion. First identified in studies of acid-induced wall loosening in cucumber hypocotyls during the 1990s, expansins are secreted into the cell wall and are thought to act as non-enzymatic proteins that enable wall extension in growing tissues.

Mechanism: Expansins do not hydrolyze polysaccharides. They appear to weaken non-covalent bonds between cellulose microfibrils and matrix polysaccharides, allowing the wall to yield when turgor pressure is applied. Their activity is enhanced by acidic pH, in line with the acid growth hypothesis linking cell wall acidification to growth responses.

Gene families and structure: The expansin superfamily includes two main subfamilies, EXPA (alpha expansins) and EXPB (beta expansins), with additional expansin-like groups EXLA and EXLB in many species. Most expansins are about 250 amino acids long, possess a signal peptide for secretion, and contain two conserved structural domains. They lack enzymatic cellulose- or hemicellulose-hydrolyzing activity.

Expression and roles: Expansin genes are highly expressed in expanding tissues such as root tips, young leaves, shoot apices, and developing fruits. They participate in processes including seed germination, organ growth, fruit softening, and abscission. Hormones like auxin, gibberellins, and ethylene regulate expansin expression, and environmental factors can influence their activity.

Relevance and applications: Elucidating expansin function informs understanding of plant growth and cell wall biomechanics. In agriculture, manipulation of expansin activity has potential to modify plant architecture, fruit texture, and harvest timing, as well as to affect biomass deconstruction for bioenergy.